An alkynylaminomethylaluminum species with a six-membered chair conformation Al2C2N2 framework: A new path to geminal N/Al frustrated lewis pairs

Article abstract of DOI:10.1021/om200967e

The hydroalumination of the fused pentahydride aminocarbaaluminum species [(C₆H₃(iPr₂-2,6)N(μ-AlH₂)CH ₂)₂(μ-AlH)NMe₃] (1) with phenylacetylene resulted in the unexpected dimer [C₆H₃(iPr ₂-2,6)(*N)HCH₂Al(CCPh)₂]₂ (2) with chiral centers at the nitrogen atoms. Compound 2 was characterized by X-ray structural analysis and contains geminal frustrated Lewis pairs.

Full text of DOI:10.1021/om200967e

Note
pubs.acs.org/Organometallics
An Alkynylaminomethylaluminum Species with a Six-Membered
Chair Conformation Al2C2N2 Framework: A New Path to Geminal N/Al
Frustrated Lewis Pairs
Wenjun Zheng,*,† Chengfu Pi,‡ and Haishun Wu†
†Institute of Organic Chemistry, Shanxi Normal University, Gongyuan Street 1, Linfen, Shanxi Province 041004, People’s Republic of
China
‡Qianjiang College, Hangzhou Normal University, Hangzhou 310012, People’s Republic of China
S
* Supporting Information
ABSTRACT: The hydroalumination of the fused pentahydride aminocarbaaluminum species
[(C6H3(iPr2-2,6)N(μ-AlH2)CH2)2(μ-AlH)NMe3] (1) with phenylacetylene resulted in the
unexpected dimer [C6H3(iPr2-2,6)(*N)HCH2Al(CCPh)2]2 (2) with chiral centers at the
nitrogen atoms. Compound 2 was characterized by X-ray structural analysis and contains
geminal frustrated Lewis pairs.
related structural conformations seemed to be required for
advances in this area. Herein we report the synthesis and
structural characterization of the dimeric alkynylaminomethy-
laluminum frustrated Lewis pair [C6H3(iPr2-2,6)(*N)HCH2Al-
(CCPh)2]2 (2) with two chiral centers at nitrogen atoms
stabilized with bulky C6H3(iPr2-2,6) groups, which was
prepared by the hydroalumination of the fused amino-
carbaaluminum hydride [(C6H3(iPr2-2,6)N(μ-AlH2)CH2)2(μ-
AlH)·NMe3] (1)5 with phenylacetylene.
rganometallic complexes of the type [RmMCH2XR′n]x
with heteroatoms in positions geminal to the metals (R,
O
R′ = organic groups; M = B, Al, Ga, In, Li, Mg; X = N, P, S, O)1
have been demonstrated to present carbenoid reactivity2 and
have shown potential as new reagents for synthetic
applications.3 By the metathesis reaction of Me2NCH2Li (or
MeSCH2Li) with Me2MCl (M = Al, Ga, In), a series of geminal
dimethylaminomethyl and methylthiomethyl compounds of
aluminum, gallium, and indium with the formulas
(Me2NCH2MMe2)2 and (MeSCH2MMe2)2 have been prepared
by Mitzel and co-workers, respectively.1b−d These compounds,
investigated by X-ray diffraction analysis, present exclusively
dimeric cyclohexane-like chair conformations in the solid state.
Further investigations showed that the dimers seemed to be
retained in all phases; however, M−S bond cleavage in the six-
membered rings of (MeSCH2MMe2)2 occurred to give
monomers bound to the Lewis base via the metal atom by
the addition of Lewis bases stronger than Me2S.1b Recently, the
synergic action of sterically hindered geminal frustrated Lewis
pairs (FLPs) such as (R2PCH2AlMe2)2 and R2PCH2CH2B-
(C6F5)2 have attracted great interest in activating CO2, H2, and
many small organic molecules in the varied organic trans-
formations.4 For example, the stable Lewis adducts
(R2PCH2AlMe2)2 (R = Me, Ph) reacted with CO2 to afford
the corresponding aluminum carboxylates,4d while
(C6F5)2PCH2CH2B(C6F5)2 underwent 1,2-addition reactions
to an alkene (alkyne) or a 1,3-addition reaction to mesityl azide,
respectively, to give the corresponding addition products.4e
Therefore, an investigation of the synthetic strategies of the
geminal group 13 element frustrated Lewis pairs as well as the
RESULTS AND DISCUSSION
■
The synthetic chemistry is shown in Scheme 1. The reaction of
dry phenylacetylene with 15 gave, after workup, the unexpected
dimeric alkynyl aminomethylaluminum compound [C6H3(iPr2-
2,6)(N*)HCH2Al(CCPh)2]2 (2) (N* = chiral center) as a
moisture- and oxygen-sensitive colorless white solid in 65%
isolated yield as well as the known aluminum acetylide complex
[(PhCC)3Al·NMe3] (3).6 Complex 2 is readily soluble in
THF, DMSO, warm toluene, and benzene but is sparingly
soluble in n-hexane.
The solid-state structure determined by X-ray diffraction
analysis showed that the structure of 2 adopts a dimeric
Al2C2N2 ring configuration (Figure 1),7 which is similar to
those found in (Me2NCH2MMe2)2 and (MeSCH2MMe2)2
previously reported by Mitzel et al. (M = Al, Ga, In).1b−d
The dimeric 2 has a crystallographic inversion center at the
Received: October 11, 2011
Published: May 9, 2012
© 2012 American Chemical Society
4072
dx.doi.org/10.1021/om200967e | Organometallics 2012, 31, 4072−4075

Organometallics
Note
Scheme 1. Preparation of the Dimeric Alkynylaminomethylaluminum Frustrated Lewis Pair [C6H3(iPr2-
a
2,6)N*HCH2Al(CCPh)2]2 (2)
a
N* = chiral center.
membered Al2C2N2 core of 2, the aluminum atoms are
connected exclusively to carbon and nitrogen atoms, and the
coordination environments of aluminum are almost identical.
The acetylide groups at aluminum atoms display a terminal
feature which is comparable to that found in the complex [(3,5-
tBu2pz)(μ4-Al(CCPh)2)]2.8 The most interesting finding is
that one hydrogen atom is located on each nitrogen atom and
the nitrogen centers thereby become chiral in 2. Thus, the
hydrogen atoms at the adjacent two prochiral carbon atoms
must be diastereotopic. The Al−C bond distance (1.996(2) Å)
within the ring is somewhat shorter than those reported in
(iPr2NCH2AlMe2)2 (Al−C = 2.007(2) Å)1d but is quite
comparable to that found in the complex (Me2NCH2AlMe2)2
(Al−C= 1.980(1) Å),1c whereas the Al−C bond lengths in the
Al−CCPh groups are within the range of 1.918(2)−1.926(2)
Å, close to those found in [(3,5-tBu2pz)(μ4-Al(CCPh)2)]2
(1.913(3)−1.929(3) Å).8 The Al(1)−N(1) (1.990(2) Å) and
N(1)−C(29A) bond lengths (1.530(2) Å) are close to the Al−
N and N−C distances associated with (iPr2NCH2AlMe2)2 (Al−
N = 2.042(1) Å; N−C = 1.521(2) Å), indicative of N−C
single-bond character.1d The N−C−Al (113.64(13)°) and C−
N−Al angles (109.29(13)°) of the core are more acute than
those found in (Me2NCH2AlMe2)2 (N−C−Al = 121.0(1)°, C−
N−Al = 110.7(1)°)1c but are somewhat larger than those found
in 1 (C−N−Al (108.68(10)° (av)) and the ideal tetrahedral
angle, likely due to a somewhat strain-free environment of the
six-membered ring.
Figure 1. Molecular structure of 2 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Al(1)−C(1) = 1.918(2), Al(1)−
C(9) = 1.926(2), Al(1)−N(1) = 1.990(2), Al(1)−C(29) = 1.996(2),
N(1)−C(17) = 1.477(3), N(1)−C(29A) = 1.530(2); C(1)−Al(1)−
C(9) = 118.21(11), C(1)−Al(1)−N(1) = 107.26(9), C(9)−Al(1)−
N(1) = 110.35(9), C(1)−Al(1)−C(29) = 111.82(9), C(9)−Al(1)−
C(29) = 105.51(10), N(1)−Al(1)−C(29) = 102.60(8), C(17)−
N(1)−C(29A) = 116.16(15), C(17)−N(1)−Al(1) = 123.00(13),
C(29A)−N(1)−Al(1) = 109.30(12), C(17)−N(1)−H(1) =
101.4(14), C(29A)−N(1)−H(1) = 104.7(14), Al(1)−N(1)−H(1) =
98.3(15). Symmetry code: (A) −x, −y + 2, −z + 1.
The elemental analysis results are in good agreement with
1
the formula of 2. The H NMR (C6D6, 23 °C) spectrum of 2
middle of the six-membered Al2C2N2 ring formed by Al−N
donor−acceptor bonds. The ring adopts a chair conformation
in the solid state with the bulky C6H3(iPr2-2,6) groups on the
nitrogen atoms being in equatorial positions. The arrangement
of the six-membered-ring conformation being significantly
different from that found in 15 suggests that the molecular
rearrangement occurred during the transformation. In the six-
1
exhibits four sharp doublets at δ 1.56 (6 H, J = 4.0 Hz), 1.41
(6 H, 1J = 4.0 Hz), 1.31 (6 H, 1J = 8.0 Hz), and 1.17 (6 H, 1J =
8.0 Hz) for the −CH3 units of the isopropyl groups and two
septets at δ 4.76 and 4.10 ppm for CH(CH3)2 groups, probably
indicating the molecular asymmetry at the exocyclic groups and
a hindered rotation about the exocyclic N−C bond in the
4073
dx.doi.org/10.1021/om200967e | Organometallics 2012, 31, 4072−4075

Organometallics
Note
solution. Notably, only a single doublet at δ 2.43 (4 H) was
observed, which is probably attributable to the two
diastereotopic hydrogen atoms at the prochiral carbon
(−CH2) of the core. This observation is reminiscent of the
signals found in 1 as well as in the previously reported
polyhedral aluminum species [{(μ3-AlH)(μ3-CH2NtBu)}4]
(4),9 where two sets of coupled doublets are observed at δ
2.569 (2 H) and 2.430 (2 H) (1J = 13.60 Hz, ratio 1:1) for 1
and at δ 1.61 (4 H) and 1.85 (4 H) (1J = 12.65 Hz, ratio 1:1)
for 4,9 respectively. The two doublets for 1 and for 4 had been
tentatively assigned to the two nonequivalent hydrogen atoms
CHaHb (a ≠ b) of the carbon due to the constraint of the three
rings of 15 and the ring of the polyhedral 4.9 However, the
single doublet for 2, as a reviewer suggested, is more likely due
to diastereotopic hydrogen atoms at the adjacent prochiral
carbon −CHaHb (a ≠ b) of the core rather than the constraint
of the ring.5,9 According to the integration, the single doublet
for −CHaHb observed for 2 is likely due to two sets of
overlapped doublets with a small difference of chemical shifts.
We attempted to elucidate preliminary information regarding
the occurrence of a single doublet for the −CH2 groups from
atoms. The successful preparation of 2 seems to open a route to
a series of sterically hindered geminal alkynylaminomethylalu-
minum frustrated Lewis pairs by simply optimizing the
substituted groups at isocyanides5 and acetylenes. In addition,
compound 2 is expected to be suitable for further applications
such as activating small organic molecules4 and acting as a
cocatalyst in organic transformations.3c Work is proceeding
along these lines.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out under a
nitrogen atmosphere under anaerobic conditions using standard
Schlenk, vacuum line, and glovebox techniques. The solvents were
thoroughly dried, deoxygenated and distilled in a nitrogen atmosphere
prior to use. C6D6 was degassed and dried with CaH2 for 24 h before
use. Phenylacetylene was dried over molecular sieves for several days
before use. The 1H NMR and 13C NMR spectra were recorded with a
Bruker DRX-400 spectrometer. IR measurements were carried out on
a Nicolet 360 FT-IR spectrometer from Nujol mulls prepared in a
drybox. Melting points were measured in sealed nitrogen-filled
capillaries without temperature correction with a Reichert-Jung
apparatus Type 302102. Elemental analyses were carried out on an
Elemental Vario EL3 (Germany) elemental analyzer.
1
variable-temperature H NMR spectra from −60 to +60 °C in
toluene-d6. Coalescence of the diastereotopic hydrogen atoms
of 2 was, however, not observed, as the NMR chemical shift
and the contour do not change significantly, similar to what is
observed for 4.9 From the variable-temperature 1H NMR
spectra of 2 and 4, it may be assumed that compounds 2 and 4
dissociate or partially dissociate into 2a (Scheme 1) and
monomeric {(μ3-AlH)(μ3-CH2(*N)tBu)} in solution, where
the diastereotopic hydrogen atoms at the prochiral carbon are
thus nonequivalent due to the chiral nitrogen centers (the
electron lone pair on the nitrogen atom can be viewed as a
substituted group). The resonance at δ 3.60 (t, 2 H) in the 1H
NMR (C6D6, 23 °C) spectrum is attributable to the N−H
groups in 2, which is further evidenced by a medium-intensity
band (at about 3227 cm−1) in the range of N−H stretching
frequencies in the IR spectrum.
The mechanism for the formation of 2 is currently not clear
and seems complicated.10 After the metathesis reaction of 1
with phenylacetylene, the resulting aluminum species probably
undergoes dissociation into the intermediate 1a having two
intramolecular three-membered rings (CNAl) formed by weak
Al···C interactions,1c followed by hydrogen addition of
phenylacetylene to the amino groups of 1a, as shown in
Scheme 1.11 It seems more likely that in the dissociated species
the three-coordinated aluminum and nitrogen are the more
reactive sites for the addition reaction with hydrogen of
phenylacetylene to aluminum and nitrogen, where the Al−N
bond is opened, the terminal proton of the alkyne is added to
nitrogen, and the alkynido group is coordinated to aluminum.11
It is also possible that the reaction alternatively precedes a path
similar to those recently reported for Al/P and B/P systems, in
which the hydrogen atom of phenylacetylene bridges between
nitrogen and aluminum atoms (N···AlNMe3) of 1a as an
intermediate following 1,3-addition to give 2.4c,12
Preparation of [C6H3(iPr2-2,6)(*N)HCH2Al(CCPh)2]2 (2). To a
solution of 1 (0.52 g, 1.0 mmol)5 in toluene (30 mL) was slowly added
phenylacetylene (0.80 mL, 7.2 mmol) via a syringe at room
temperature. After gas evolution ceased, the volatile components
were removed under reduced pressure (0.01 mbar). The resulting
residue was dissolved in warm toluene (50 mL). The solution was
concentrated to afford 2 as white crystals at room temperature (0.54 g,
1
65% based on 1). Mp: 203−205 °C dec. H NMR (C6D6, 23 °C): δ
7.62 (d, 6 H, Ph ring), 7.10 (m, 12 H, Ph ring), 6.80 (m, 8 H, Ph ring),
4.76 (septet, 2 H, CH), 4.10 (septet, 2 H, CH), 3.60 (t, 2 H, NH),
2.43 (d, 2 H, CH2), 1.56 (d, 3J = 4.0 Hz, 6 H, CH3), 1.41 (d, 3J = 4.0
Hz, 6 H, CH3), 1.31 (d, 3J = 4.0 Hz, 6 H, CH3), 1.17 (d, 3J = 4.0 Hz, 6
H, CH3), 13C{1H} NMR (C6D6, 23 °C): δ 132.4, 132.3, 132.2, 128.5,
127.3, 126.8, 125.3, 124.9, 124.8, 124.0 (C(Ph)), 44.4 (CH2), 29.3,
28.8 (C(iPr)), 25.6, 25.3, 24.8, 24.5 (CH3); IR (Nujol mull, cm−1):
3227 (m), 2124 (m), 1944 (w), 1873 (w), 1801 (w), 1737 (w), 1665
(w), 1597 (m), 1312 (m), 1261 (m), 1216 (m), 1161 (w), 1085 (s),
1024 (s), 921 (m), 882 (w), 802 (s), 755 (m), 728 (m), 691 (m), 603
(w). Anal. Calcd for C58H60Al2N2: C, 83.02; H, 7.21; N, 3.34. Found:
C, 82.87; H, 7.36; N, 3.23. The mother liquor was further
concentrated to give 3 as white crystals. The physical data of 3 are
identical with those reported in the literature.6
X-ray Crystallography. Suitable single crystals were sealed under
N2 in thin-walled glass capillaries. X-ray diffraction data were collected
on a SMART APEX CCD diffractometer (graphite-monochromated
Mo Kα radiation, φ−ω-scan technique, λ = 0.710 73 Å). The intensity
data were integrated by means of the SAINT program.13 SADABS14
was used to perform area-detector scaling and absorption corrections.
The structures were solved by direct methods and were refined against
F2 using all reflections with the aid of the SHELXTL package.15 The
hydrogen atoms attached to nitrogen atoms were calculated on ideal
positions. Crystallographic parameters for compound 2 along with
details of the data collection and refinement are contained in the
Supporting Information.7
In summary, the hydroalumination of the fused pentahydride
aminocarbaaluminum species [(C6H3(iPr2-2,6)N(μ-AlH2)-
CH2)2(μ-AlH)NMe3] (1) with phenylacetylene resulted in
the unexpected dimeric [C6H3(iPr2-2,6)(*N)HCH2Al-
(CCPh)2]2 (2) with chiral centers at the nitrogen atoms. The
hydroalumination in this case is probably via a process of initial
multistep intramolecular rearrangement followed by an 1,3-
addition of terminal phenylacetylene to nitrogen and aluminum
Crystal Data for 2: C58H60Al2N2, Mr = 839.04, triclinic, space group
P1, a = 11.055(5) Å, b = 11.533(5) Å, c = 11.605(5) Å, α = 75.055(5)
̅
°, β = 87.816(5)°, γ = 62.785(5)°, V = 1265.9(9) Å3, Z = 1, ρcalcd
=
1.101 Mg m−3, crystal size 0.20 × 0.12 × 0.10 mm3, F(000) = 448,
μ(Mo Kα) = 0.095 mm−1, GOF = 0.920, 4856 independent reflections
(Rint = 0.0296). The final R indices were R1 = 0.0639 (I > 2σ(I)) and
wR2 = 0.1690 (all data).
4074
dx.doi.org/10.1021/om200967e | Organometallics 2012, 31, 4072−4075

Organometallics
Note
(13) SAINTPlus Data Reduction and Correction Program v. 6.02 a,
Bruker AXS, Madison, WI, 2000.
(14) Sheldrick, G. M. SADABS, Program for Empirical Absorption
ASSOCIATED CONTENT
* Supporting Information
Tables and a CIF file giving X-ray crystallographic data for 2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
Correction; University of Gottingen, Gottingen, Germany, 1998.
̈
̈
(15) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Gottingen, Gottingen, Germany, 1997.
̈
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: wjzheng@sxnu.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support from the Program for the Top Young
and Middle-aged Innovative Talents of Higher Learning
Institutions of Shanxi (TYMIT), the Program for the Doctoral
Research Fund of the Education Ministry (Grant No. 2011
1404110001), and the National Natural Science Foundation of
China (NSFC; Grant No. 20872017).
REFERENCES
■
(1) (a) Lustig, C.; Mitzel, N. W. Chem. Commun. 2000, 1393−1394.
(b) Lustig, C.; Mitzel, N. W. Organometallics 2002, 21, 3471−3476.
(c) Lustig, C.; Mitzel, N. W. Organometallics 2003, 22, 242−249.
(d) Tian, X.; Markus, W.; Lustig, C.; Pape, T.; Frohlich, R.; Van, L. D.;
̈
Bergander, K.; Mitzel, N. W. Organometallics 2005, 24, 82−88.
(e) Tian, X.; Frohlich, R.; Mitzel, N. W. Z. Anorg. Allg. Chem. 2005,
̈
631 (8), 1442−1448 and references therein.
(2) Ruffer, T.; Bruhn, C.; Maulitz, A. H.; Strohl, D.; Steinborn, D.
̈
̈
Organometallics 2000, 19, 2829−2831.
(3) (a) Boudreau, J.; Fontaine, F.-G. Organometallics 2011, 30, 511−
519. (b) Thibault, M.-H.; Boudreau, J.; Mathiotte, S.; Drouin, F.;
Sigouin, O.; Michaud, A.; Fontaine, F.-G. Organometallics 2007, 26,
3807−3815. (c) Fontaine, F.-G.; Zargarian, D. J. Am. Chem. Soc. 2004,
126, 8786−8794.
(4) (a) Bertini, F.; Lyaskovskyy, V.; Timmer, B. J. J.; de Kanter, F. J.
J.; Lutz, M.; Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K. J. Am.
Chem. Soc. 2012, 134, 201−204. (b) Wiegand, T.; Eckert, H.; Ekkert,
O.; Frohlich, R.; Kehr, G.; Erker, G.; Grimme, S. J. Am. Chem. Soc.
̈
2012, 134, 4236−4249. (c) Appelt, C.; Westenberg, H.; Bertini, F.;
Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Angew. Chem.,
Int. Ed. 2011, 50, 3925−3928. (d) Boudreau, J.; Courtemanche, M.-A.;
Fontaine, F.-G. Chem. Commun. 2011, 47, 11131−11133. (e) Stute, A.;
Kehr, G.; Frohlich, R.; Erker, G. Chem. Commun. 2011, 47, 4288−
̈
4290. (f) Welch, G. C.; Juan, S. R. R.; Masuda, J.; Stephan, D. W.
Science 2006, 314, 1124−1126.
(5) Pi, C.-F.; Wan, L.; Wu, H-.Y.; Gu, Y.-Y.; Zheng, W.; Weng., L.-H;
Chen, Z.-X.; Wu, L.-M. Organometallics 2009, 28, 1263−1265.
(6) (a) Zheng, W.; Roesky, H. W. Dalton Trans. 2002, 2787−2796.
(b) Schiefer, M.; Reddy, N. D.; Ahn, H.-J.; Stasch, A.; Roesky, H. W.;
Schlicker, A. C.; Schmidt, H.-G.; Noltemeyer, M.; Vidovic, D. Inorg.
Chem. 2003, 42, 4970−4976.
(7) Crystallographic data for 2 are contained in the Supporting
Information.
(8) Zheng, W.; Mosch-Zanetti, N. C.; Roesky, H. W.; Hewitt, M.;
̈
Cimpoesu, F.; Schneider, T.; Stasch, A.; Prust, J. Angew. Chem., Int. Ed.
2000, 39, 3099−3101.
(9) Zheng, W.; Stasch, A.; Prust, J.; Roesky, H. W.; Cimpoesu, F.;
Noltemeyer, M.; Schmidt, H.-G. Angew. Chem., Int. Ed. 2001, 40,
3461−3464.
(10) See the Supporting Information (S-2).
(11) It has been evidenced that aluminum hydride and terminal
acetylene is prone to undergo a metathesis reaction by the elimination
of hydrogen rather than an addition reaction; see ref 6a.
(12) Momming, C. M.; Kehr, G.; Wibbeling, B.; Frohlich, R.;
̈
̈
Grimme, S.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 2414−2417.
4075
dx.doi.org/10.1021/om200967e | Organometallics 2012, 31, 4072−4075